The DE-A-1,472,189 (German laid-open patent application) describes a three-lens objective for infrared having two positive power lenses made of silicon and a negative power lens made of germanium therebetween. The two positive power lenses are menisci. The concave sides of these menisci face the shorter back or front focal length, respectively.
In this prior art objective, the spacings between the front lens and the median lens and between the median lens and the rear lens are approximately equal. All lenses are spherical. Thereby, the overall length of the objective becomes undesirably large.
The DE-A-2,519,961 also describes a three-lens objective for infrared. The three-lens objective comprises a single, positive power front lens in the form of a meniscus and made of germanium, and a rear lens element composed of two adjacent meniscus lenses convex towards the front. At least one of these rear meniscus lenses consists of germanium, as well. The rear surface of the front lens is aspherical. The remaining surfaces are spherical.
In this prior art three-lens objective, the median and rear lenses are closely adjacent.
The DE-A-3,726,355 relates to a bifocal optical infrared system in the form of a three-lens objective. The three-lens objective comprises a positive power front lens, a negative power medial lens and a rear lens having, again, positive power. The lenses consist of germanium. The rear surface of the front lens and the front surface of the rear lens are aspherical.
Also in this three-lens objective, the median and rear lenses are closely adjacent. Such closely adjacent median and rear lenses in three-lens objectives result in an increase of the overall length.
The DE-A-2,743,175 describes an infrared objective which consists of three mutually adjustable optical elements. The median and the rear optical elements, in turn, consist of two lenses each. The spacing of the median and rear optical elements is smaller than the spacing of the median optical element from the front one.
The DD-A-299,930 (German Democratic Republic patent) describes a four-lens infrared objective of the Petzval type.
A paper by J. Kross and R. Schuhmann "Zur Korrektion optischer Systeme mit aspharischen Flachen" in "Optik" Vol.70 (1985), 76-85, describes the correction of different lens systems by means of aspherical surfaces.